Preparation of photoconductive recording materials

ABSTRACT

A POROUS PHOTOCONDUCTIVE RECORDING LAYER IS OBTAINED BY DISPERSING INORGANIC PHOTOCONDUCTIVE PARTICLES SUCH AS PHOTOCONDUCTIVE ZINC OXIDE IN AN ORGANIC SOLVENT SOLUTION HAVING DISSOLVED THEREIN A MIXTURE OF POLYMERIC BINDING AGENTS FOR THE PHOTOCONDUCTIVE PARTICLES WHICH INCLUDES AT LEAST TWO POLYMERS WHICH ARE BOTH INSOLUBLE IN WATER AT PH 7, ARE SOLUBLE TO DIFFERENT EXTENTS, I.E., DIFFERENTIALLY SOLUBLE IN THE ORGANIC SOLVENT SOLUTION, FORM A CLEAR SOLUTION WITHOUT NOTICEABLE TURBIDITY WHEN SO DISSOLVED, AND ARE MUTUALLY INSOLUBLE IN ONE ANOTHER WHEN IN THE SOLID STATE; COATING THE RESULTANT DISPERSION ON A APPROPRIATE SUPPORT; DRYING THE COATING AT A RATE SUFFICIENT TO PRECIPITATE AT LEAST A PORTION OF ONE POLYMER PRIOR TO TH EPRECIPITATION OF THE OTHER POLYMER; AND THEN COMPLETING THE DRYING OF THE COATING. AS A RESULT OF THE PREFERENTIAL PRECIPITATION OF THE LESSER SOLUBLE POLYMER, THERE RESULTS A DRIED COATING STRUCTURE CONTAINING RANDOM INTERCONNECTING MICROSCOPIC CAVITIES.

27, K. E. VERHIL-Li ETA!- 3,595,591

PREPARATION OF PHOTOCONDUCTIVE RECORDING MATERIALS Filed April 26, 1968 3 Sheets-Sheet 1 July 27,1971 V R ETAL 3,595,591

3 Sheets-Sheet 2 W Qu w m y 27, 1971 K. E. VERHlLLE ETAL 3,5

PREPARATION OF PHOTOCONDUCTIVE RECORDING" MATERIALS Filed A ril 26, 1968 a Sheets-Sheet :s

0 Q o v (\f') O Q) L 1 1 l n 1 Q 0 n \r "1 N T Q (S Q 6 Q Q' Q US. Cl. 117-401 17 Claims ABSTRACT (NF THE DlISCDOSURlE A porous photoconductive recording layer is obtained by dispersing inorganic photoconductive particles such as photoconductive zinc oxide in an organic solvent solution having dissolved therein a mixture of polymeric binding agents for the photoconductive particles Which includes at least two polymers which are both insoluble in Water at pH 7, are soluble to different extents, i.e., differentially soluble in the organic solvent solution, form a clear solution without noticeable turbidity when so dissolved, and are mutually insoluble in one another when in the solid state; coating the resultant dispersion on an appropriate support; drying the coating at a rate suflicient to precipitate at least a portion of one polymer prior to the precipitation of the other polymer; and then completing the drying of the coating. As a result of the preferential precipitation of the lesser soluble polymer, there results a dried coating structure containing random interconnecting microscopic cavities.

The present invention relates to porous improved photo conductive recording materials, which contain an inorganic photoconductor dispersed in a binder and which are suited for use in electrophotography, and also relates to the preparation of such materials.

A known type of electrophotographic recording plate or sheet comprises a relatively conductive backing, which is coated with a photoconductive insulating composition prepared by intimately mixing and grinding together a photoconductive insulating pigment, a binder of high electrical resistivity, and one or more solvents for the binder.

In photoconductive binder coatings, photoconductive zinc oxide is preferably used as the dispersed photoconductive substance on account of its high photo-sensitivity, its susceptibility to spectral sensitization, and its bright white colour. The binder must normally be electrically insulating and its binding power must be as high as possible. Consequently the binding agents should possess optimal mechanical and electrical properties at the same time.

The photoconductive substance together with the binder and other additives, constitutes a paint, which can be applied to a suitable support, e.g. of paper, by conventional coating techniques such as brush-, spray-, knife-, and dip-coating.

It is known from Angew. Chem. 72 (1960) No. 19/20, pp. 730-738, that the production of charge carriers and the dissipation of charges on a zinc oxide-containing photoconductive layer depend on the chemisorption and desorption of oxygen, and that it is possible to reduce the conductivity in the dark of the zinc oxide/ binder coating by a decrease in contact points between the separate zinc oxide grains. For obtaining a highly Water-repellent and highly insulating recording layer a silicon resin can be used as binding agent.

Disadvantages associated with such binding agent include a somewhat low inherent sensitivity (i.e. dischargeability by exposure to light), requiring intense exposures of the recording layer, and difiiculty in providing spectral States Pate 3,595,691 Patented July 27, 1971 sensitization. Moreover, the molecular weight of the binding agent is usually not sufiiciently high as to prevent sticking, and special care therefore must be taken during drying to prevent sticking.

It has now been found that porous photoconductive zinc oxide binder coatings having a high chargeability in the dark, a surprisingly high sensitivity even without use of spectrally sensitizing agents, and good mechanical strength can be prepared by a process comprising the steps of (1) dispersing photoconductive zinc oxide grains in a solution comprising at least one organic solvent having dissolved therein at least two dissolved polymers having the characteristics as described hereafter; (2) coating the dispersion on a suitable support and (3) drying the coating at such a rate that at least a part of one of the polymers precipitates from the solvent(s) before the other polymer(s), and that on completion of the drying, the dried coating contains randomly interconnecting microscopic cavities. By the term microscopic is meant that the cavities are not discernible to the unaided eye. It is preferred that the average width of the cavities is in the range from 2 to 12 microns.

The cavities should not take up more than 50% by volume of the layer.

The polymers used according to the invention may be homopolymers or copolymers, and have the following characteristics (a) they are insoluble in water at pH 7;

(b) they are differentially soluble in the solvents in which they are dissolved;

(c) when dissolved in said solvent(s), they form a clear solution, i.e. without any noticeable turbidity in the solution from which the coating is to be applied;

(d) they are not mutually soluble in the solid state, by

which is meant that when these polymers are precipitated from a solution containing only said polymers they are precipitated to form a mass, for instance a layer consisting of said polymers containing macroscopic heterogeneities, which are detectable by the unaided eye.

The optimal concentration of the mixture of said polymers in the solution, i.e. the concentration for obtaining maximum sensitivity of the recording layer at minimum loss in optical density, may be determined by a test in which a precipitating agent is used whereby the solubility of the polymers in the solvent is decreased, thereby simulating the effect of the drying step in the process of the invention. The optimal concentration for a given ratio by weight of the different polymers (see FIGS. 1, 2 and 3) is found at maximum variation of the 'y-factor, which is determined according to the ratio:

The precipitant should be a liquid wherein neither or none of the said polymers is soluble and which is completely miscible with the solvent mixture wherein the said polymers are dissolved. The number of milliliters of precipitant used in the above described test corresponds with the amount necessary to reach the turbidity point in the titration of a given number of milliliters of solution of the said polymers in the said solvent mixture.

The drying of the coating prepared according to the invention is preferably carried out as a two stage drying process, in which in the first drying stage the temperature is kept relatively low e.g. below 30 (3., preferably in the neighbourhood of room temperature (20-25 C.), the rate at which the solvent(s) are evaporated in the said first drying stage being controlled so that the layer, before being dried in the second stage, loses solvent(s) by evaporation sulficiently to ensure at least a partial precipitation of one of the polymers due to the concentration of the solution surpassing the solubility limit of the said polymer. This aim is most easily achieved when the original dispersion is concentrated to a point just slightly less than than the concentration at which the said polymer is present in a saturated solution.

In the first drying stage it is advantageous to use a slow velocity drier, preferably a laminar current drier known as a tangential current drier wherein an air current stril es parallel along the coated layer. The rate of evaporation produced by such a drying system is less than that produced by a high-velocity drier (impinging air current drier), wherein aid perpendicularly impinges from nozzles to the coated layer.

In the second drying stage, wherein advantageously but not necessarily an impinging air current drier is used, the temperature of the impinging air may be raised well above room temperature, for instance up to 10 to 20 C. below the boiling point of one of the solvents.

The average width of the interconnecting microscopic cavities of the dried layer can be influenced by prolonging or shortening the time of the first drying stage. A prolongation of this period increases the average width of the cavities and increases the sensitivity of the material, whereas shortening of this period gives rise to a less porous material having a lower sensitivity.

A possible explanation of the formation of the interconnecting microscopic cavities produced according to the invention in zinc oxide containing layers is that the most soluble polymer reticulates upon drying during the second drying stage when it comes into contact with the first precipitated binding agent. I

Particularly suitable binding agent systems for preparmg recording materials according to the present invention can be found in the combination of polymers with different electrophilic character, e.g. a combination of polymers wherein one type (Group A) contains groups having the properties of a Lewis base (i.e. substances that provide unshared electron pairs available for co-ordination), e.g. ether groups, amino groups, or halogen atoms and another type (Group B) which comprises groups with coordinated electron pairs, thus a neutralized Lewis base or acid, e.g. an ester group, or groups having the properties of a Lewis acid (i.e. substances capable of co-ordinating with unshared electron pairs), e.g. carboxylic anhydride groups such as maleic anhydride groups.

Polymers, which illustrate the nature of the binding agents suited for use in combination according to this invention, are illustrated in the following table, without however, limiting the invention thereto.

GROUP A Poly(vinyl ether) compounds e.g. copoly(vinyl methyl ether maleic anhydride);

Cellulose ether compounds e.g. ethylcellulose and ethylallylcellulose;

Vinyl acetal polymers e.g. poly(vinyl-n-butyral);

Halogen-containing polymers e.g. vinyl chloride polymers and copolymers e.g. copoly(vinyl chloride-vinyl acetate) e.g. in a ratio of 80/20, copoly(vinyl chloride-vinyl acetate-maleic anhydride), e.g. in a ratio of 85 14/1 a copolymer of vinylchloride, vinylacetate, and a minor amount (less than mole percent) of an unsaturated bivalent dicarboxylic acid e.g. maleic acid, citraconic acid and itaconic acird, copoly (ethylene-vinylsulfonylchloridevinylchloride) chlorinated rubbers e.g. chlorinated natural rubber, chlorinated polypropylene.

GROUP B Aliphatic esters derived from poly(vinyl alcohol) e.g. poly(vinyl acetate) and copolyrners of vinyl acetate wit higher (C -C aliphatic carboxylic esters of vinyl alcohol e.g. copoly(vinyl avetate-vinyl laurate) (/20).

Polyacrylate and polymethacrylate ester compounds e.g. polymethylacrylate, polyethylacrylate, poly(methyl methacrylate), poly(isobutyl methacrylate), copoly (styrene-butylacrylate) In the mixture of above-cited polymers more than two binding agents may be present.

In the preparation of the electrophotographic recording materials according to the present invention it is important to use a solvent or solvent mixture wherein both types of polymers are sufliciently differently soluble and wherefrom preferably the polymer with the Lewis base groups first precipitates upon evaporation of the solvent(s). In this connection preferably use is made of a solvent mixture containing at least two miscible solvents, one of which has a higher vapour pressure at the drying temperature then the other and wherein the solvent having the highest vapour pressure is a good solvent for the polymer with the Lewis base groups, whereas the solvent having a lower vapour pressure is a better solvent for the other polymer.

The photoconductive dispersion can be coated according to standard techniques e.g. by dipping, by pouring, the pigment-binder mixture onto the support to be coated, or by applying the mixture with a dip-roller.

The dried recording layer 8 may have a thickness rang- 1ng from 1 to 30 microns, preferably from 5 to 15 microns, and the proportion of pigment to binder may be within the range between 0.5 and 10 parts, preferably between 1 and 3.5 parts of pigment calculated on 1 part by weight of binder. The ratio of polymer(s) of group (A) and polymer(s) of group (B) is chosen taking into account the need for a sufiicient mechanical strength and a light-sensitivity as high as possible.

A preferred electrophotographic material, the recording element of which is prepared by the above described steps, contains a mixture of ethylcellulose and copoly (vinyl acetate/vinyl laurate) (80/20) as binder in a weight ratio of 4:1 to 1:4.

A suitable combination of solvents for the binder composition containing ethylcellulose and copoly(vinyl acetate/vinyl laurate) (80/20) contains approximately 83 parts by volume of sym.-dichloroethane, 7 parts by volume of ethanol and 10 parts by volume of a solvent containing a ketone group e.g. cyclohexanone or ethyl methyl ketone.

The photoconductive recording layers prepared according to the present invention may contain, in addition to the photoconductive substance(s) and the binder, spectrally sensitizing agents e.g. those described in the United Kingdom patent specification 1,020,504 and in the published Dutch patent applications 6704706 and 6706575.

Further they may contain compounds increasing the dark-resistivity of photoconductive zinc oxide, e. g. the acid compounds described in the Belgian patent specification 612,102. When selecting coating aids e.g. dispersing agents, preferably those are chosen, which reduce the dajrik-resistivity of the photoconductive layer the least possi e.

The supports or base materials are chosen in view of the specific charging, exposing, recording, developing and/ or transfer technique wherein the recording material is to be used.

In electrophotographic recording techniques wherein the photoconductive layer is electrostatically charged, the support preferably has an electric volume resistivity considerably lower than that of the recording layer. Suit able supports are described in the United Kingdom patent specifications 1,020,504 and 995,491.

bf'k paper support of the glassine type is used prefera y.

The photoconductive layer of an electrophotographic material prepared according to the present invention can be used 111 recording systems wherein prior to the image;

wise exposure an electric charge is non-dilferentially applied according to known methods.

The material can, however, also be used in recording techniques wherein the exposure step precedes the charging step. Such techniques are described eg in the United Kingdom patent specifications 1,033,419 and 1,033,420.

The following examples illustrate the present invention without however limiting it thereto.

Example 1 The following mixture was ground in a sand mill at a rate of 24 litres per hour:

7.5 g. of Vinnapas B100/20VL, which is a copoly(vinyl acetate/ vinyl laurate) (80/ 20) sold by Wacker Chemie GmbH, Burghausen, Germany,

7.5 g. of Pioloform BL 18, a poly(vinyl-n-butyral) sold by Wacker Chemie GmbH, Burghausen, Germany,

208.5 ccs. of sym.-dichloroethane,

30 cos. of cyclohexanone,

17.5 ccs. of ethanol,

3.25 ccs. of a solution of acid butyl phosphate in ethanol,

3.25 ccs. of a 10% solution of tetrachlorophthalic anhydride in ethanol,

75 g. of photoconductive zinc oxide prepared by oxidation of zinc vapour.

The following sensitizing dyes were then thoroughly blended with the milled composition:

3 cos. of a 0.25 solution in ethanol of Orange Acridine Brillante EZ, sold by Prancolor, Brussels, Belgium (C.I. 46,005

3.4 ccs. of a 0.20% solution in ethanol of a compound corresponding to the following structural formula:

@013. l /OZHS N 02115 3.75 ccs. of a 1% solution in ethanol of Eriochrome Cyanine R (C.I. 43,820) sold by Geigy, Basel, Switzerland.

The resulting dispersion was coated onto a glassine type paper support at a rate of 30 g. per sq. m. by means of a dip-coating device.

The drying of the coated layer proceeded in the following manner; in the first drying stage the coated material was transported through a tangential current drier (air speed 2 m./sec.) at 20 C. over a period of 5 seconds. In the second drying stage the coated material was led till dry through an impinging air current drier (air speed 8 m./sec.) at a drying temperature of 60 C. A mechanically very strong recording layer, which exhibited interconnecting microscopic cavities of an average width of approximately 4 1. was obtained.

The accompanying table contains the relative sensitivity values of a recording layer as described in the present example and of recording layers differing therefrom by the presence of only one binder of the mentioned composition.

Binder: Relative sensitivity (A) Pioloform BL 18 (trade name) 1 (B) Vinnapas B100/20VL (trade name) 4 Mixture of (A) and (B) as described in the example 12 The 'y-factor depending on the concentration of the mixture of polymers (A) and (B) in the used solvent mixture is shown in FIG. 1 of the accompanying drawings, in which the concentration is shown on the abscissa and the 'y-factor on the ordinate.

The concentration (C) is expressed by the grams of copoly(vinyl acetate/vinyl laurate) (80/20) dissolved in 100 cos. of the solvent mixture used in coating the zinc oxide dispersion of Example 1, together with the necessary grams of the other polymer (B) to maintain the weight ratio of the polymers as described above, viz 1:1.

The value Co shown in the curve of FIG. 1 stands for the grams of copoly (vinyl acetate/ vinyl laurate) (80/ 20) used in the present example viz 2.75 g. per 100 cos. of the solvent mixture used in the present example.

For determining the y-value n-hexane was used as precipitant.

Example 2 The constituents of the binder described in Example 1 were changed in quantity and nature:

3.75 g. of Vinnapas B100/20VL (trade name) were used instead of 7.5 g. and the polyQvinyl-n-butyral) was replaced by 3.75 g. of a cellulose derivative corresponding to the following structural formula:

(degree of substitution (DS) 2.25 for the ethyl groups and 0.06 for the allyl groups).

When coating and drying as described in Example 1, analogous results were obtained.

Example 3 1.275 litre of a 20% solution of iVnnapas B100/20VL (trade name) in sym.-dichloroethane was mixed With a solution of 120 g. of Ethylcellulose K 50 in 3.705 liters of sym.-dichloroethane, 290 ccs. of ethanol, and 600 ccs. of cyclohexanone. The type of ethylcellulose used is sold by Hercules Powder Co. Inc., Wilmington, Del., U.S.A. A 5% by weight solution of this type of polymer in a mixture (80:20 parts by volume of toluene and ethanol possesses at 25 C. a viscosity range of 40-52 cps.) (degree of substitution of ethyl groups: 2.282.38).

The following compounds were then added:

acid butyl phosphate, 10% solution in ethanol-65 ccs.

tetrachlorophthalic anhydride, 10%- solution in ethanol-- 65 ccs.

zinc oxide as used in Example 11,500 kg.

The mixture was well blended whereafter the whole dispersion was sand milled at a rate of 24 litres/hour.

The resulting dispersion was spectrally sensitized with 60 cos. of a 0.25% solution in methanol of (Cl. 46,005) Orange Acridine Brillante EZ sold by Francolor, Brussels, Belgium and ccs. of a 1% solution in methanol of Eriochrome Cyanine R (Cl. 43,820) sold by Geigy A.G., Basel, Switzerland.

The dispersion was coated Within 8 hours after completing the formula at a rate of 35 g. of solids per sq. m. The drying proceeded in two stages as in Example 1: the first drying stage in a tangential drier at 25 C. with an air rate of 2 m./ sec. for 20 sec. and the second drying stage in an impinging air drier at 50 C. with an air velocity of 8 m./sec. through the nozzles. The layer was thoroughly dried in this zone.

The dried layer possessed the characteristic structure with interconnecting microscopic cavities.

The 'y-factor as a function of polymer concentration is shown in FIG. 2.

The concentration (C) is expressed by the grams of copoly(vinyl acetate/vinyl laurate) (/20) dissolved in ccs. of the solvent mixture used for coating the zinc oxide dispersion of Example 3, together with the neces- 7 sary grams of the other polymer to maintain the ratio of the polymers as in Example 3.

The value Co shown in the curve of FIG. 2 stands for the grams of copoly (vinyl acetate/ vinyl laurate) (80:20) used in Example 3, viz 4.2 g. per 100 ccs. of the solvent mixture used in that example.

For determining the 'yvalue, n-hexane was used as precipitant.

Example 4 In Example 3 the ethylcellulose was replaced by Ethylcellulose G50 (sold by Hercules Powder Co. Inc.,

Wilmington, Del., U.S.A.), having a viscosity range of 40-52 cps. in a 5% by weight concentration at 25 C. in a mixture of toluene/ ethanol (80:20 parts by volume) and having a degree of substitution of ethyl groups: 2.21-2.28.

The results were entirely similar to those obtained according to the procedure of Example 3.

Example 5 2.4 kg. of Ethylcellulose N7 (trade name of an ethylcellulose sold by Hercules Powder Company Inc., Wilmington, Del., U.S.A.) was dissolved in 80 litres of sym.-dichloroethane (a 5% by weight solution of this type of ethylcellulose in a mixture of toluene and ethanol (80:20 parts by volume) possesses at 25 C. a viscosity range of 68 cps.) degree of substitution of ethyl group: 2.50-2.66.

The resulting solution was diluted with 3 litres of ethanol and 3 litres of cyclohexanone. Subsequently 60 kg. of Zinc oxide as used in Example 1 were added while mixing. The whole dispersion was fed through a homogenizer till all agglomerates were sufficiently broken. The following compounds were then added to the dispersion:

58.8 litres of sym.-dichloroethane,

8.6 litres of ethanol 10.2 kg. of Vinnapas B100/20VL (trade name) dissolved in 20.4 litres of sym-dichloroethane,

2.58 liters of a solution in ethanol of acid butyl phosphate,

2.58 liters of a 10% solution in ethanol of tetrachlorophthalic anhydride,

7.8 kg. of Ethylcellulose N7 (trade name) dissolved in 39 liters of sym.-dichloroethane,

2.4 liters of a 0.25% solution of Orange Acridine Brillante EZ (C.I. 46,005) in methanol,

3 liters of a 1% solution in methanol of Eriochrome Cyanine (CI. 43,820),

21 liters of ethyl methyl ketone.

The composition was coated at a rate of 32 g. of solids per sq. m. of a glassine type paper base, which is inherently sufficiently resistant to penetration of the solvents used.

The drying time in the first drying stage was 10 seconds at a tangential air rate of 4 m./sec. at C. In the second drying stage the temperature was raised to 70 C.

The sensitivity of this material was 4 times higher than that of the corresponding material obtained with the exclusive use of an impinging air drier working at 80 C.

Example 6 18.88 liters of sym.-dichloroethane were mixed with 1.664 liters of ethanol and 11.136 kg. of a 20% solution of Vinnapas B100/20VL (trade name) in sym.-dichloroethane. After thorough stirring 3.20 liters of ethyl methyl ketone and 1.024 kg. of Hostalit M 131 (trade name of a copolymer of vinyl chloride (:87 mole percent), vinyl acetate and minor amounts (less than 1 mole percent) of an anhydride of an unsaturated dicarboxylic acid, a 20% solution in ethylacetate at 20 C. has a viscosity of 120 (H2)4SO2NHCOCH3 (OH2)4S02NH-C 0 CH3 and 63 ccs. of a 1% solution of Bromophenol blue in methanol were added. The coating and drying procedure of Example 5 was then followed with comparable results.

The v-factor as a function of polymer concentration is shown in FIG. 3.

The concentration (C) is expressed by the grams of copoly(vinyl acetate/vinyl laurate) (/20) dissolved in ccs. of the solvent mixture used for coating the zinc oxide dispersion of Example 6, together with the necessary grams of the other polymer to maintain the ratio of the polymers used in Example 6.

The value Co shown in the curve of FIG. 3 stands for the grams of cop0ly(vinyl acetate/vinyl laurate) (80/20) used in Example 6, viz 4.8 g. per 100 cos. of the solvent mixture used in that example.

For determining the y-value, n-hexane was used as precipitant.

We claim:

1. A method of producing a porous photoconductive recording material, which comprises the steps of (1) dispersing inorganic photoconductive particles in a solution comprising at least one organic solvent having dissolved therein a mixture of polymeric binding agents including at least two polymers characterized by:

(a) insolubility in water at pH 7;

(b) differential solubility in said solvent in which they are dissolved;

(c) when dissolved in said solvent, formation of a clear solution without any noticeable turbidity in said solution;

(d) absence of mutual solubility in the solid state, whereby when said polymers are precipitated from a solution thereof they form a polymer mass containing macroscopic heterogeneities, which are detectable by the unaided eye.

(2) coating the dispersion of a support, and

(3) drying the coating at such a rate that at least a portion of one of the polymers precipitates from said solvent before the other polymer whereupon completion of the drying, the dried coating contains random microscopic cavities, said cavities constituting not more than 50% by volume of the layer.

2. A method of producing a porous photoconductive recording material according to claim 1, wherein the inorganic photoconductive particles are photoconductive zinc oxide particles.

3. A method according to claim 1 wherein the rate of drying is such as to provide the resulting microscopic cavities with an average width of 2 to 12 microns.

4. A method according to claim 1, wherein the drying step comprises a two-stage drying process, having a first drying stage wherein the temperature is kept relatively low, the rate at which the solvent(s) are evaporated in the said first stage being controlled so that the layer before being dried in the second stage, loses a sufilcient amount of such solvent by evaporation to ensure at least a partial precipitation of one of the polymers due to the concentration of the solution surpassing the solubility limit of the said polymer, and a second drying stage wherein drying proceeds at a relatively higher rate and at a temperature higher than that reached in the first drying stage.

5. A method according to claim 4, wherein in the first drying stage a laminar current drier is used and in the second drying stage an impinging current drier is used.

6. A method according to claim 1, wherein a mixture of polymers is used selected from the following groups A and B group A:

polyvinyl acetals polyvinyl ethers cellulose ethers halogen-containing vinyl polymers, chlorinated rubbers, and chlorinated polyolefins group B:

aliphatic esters of polyvinyl alcohol polyacrylate and polymethacrylate esters.

7. A method according to claim 1, wherein a mixture of polymers consisting of a cellulose ether and an aliphatic ester of polyvinyl alcohol is used.

8. A method according to claim 7, wherein an ethylcellulose ether and a vinyl acetate/vinyl laurate copolymer is used.

9. A method according to claim 7, wherein an ethylallyl cellulose ether and a vinyl acetate/ vinyl laurate copolymer is used.

10. A method according to claim 6, wherein a mixture of polymers is used consisting of a polyvinyl acetal and a vinyl acetate/vinyl laurate copolymer.

11. A method according to claim 6, wherein a combination of polymers is used consisting of a copolymer of vinyl chloride, vinyl acetate, and a minor amount of 10 a bivalent unsaturated carboxylic acid anhydride and a vinyl acetate/vinyl laurate copolymer.

12. A method according to claim 1, wherein a spectrally sensitizing dye is added to the photoconductive dispersion.

13. A method according to claim 1, wherein the photoconductive dispersion is applied to a glassine type paper support.

14. A method according to claim 1, wherein a photoconductive recording layer having a thickness ranging from S to 30 microns is produced.

15. A method according to claim 1, wherein the proportion of photoconductive pigment to the binder ranges from 0.5 to 10 parts by weight calculated on 1 part by weight of binder.

16. A method according to claim 1, wherein the binder contains ethylcellulose and vinyl acetate/ vinyl laurate copolymer (/20) in a weight ratio of 4:1 to 1:4.

17. A method according to claim 16, wherein the solvent system used for coating the photoconductive dispersion contains approximately 83 parts by volume of sym.-dichloroethane, 7 parts by volume of ethanol, and 10 parts by volume of a solvent containing a ketone group e. g. cyclohexanone or ethyl methyl ketone.

US. Cl. X.R. 

